1. Field of the Invention
The invention relates to cellular radio systems and, more particularly, to cellular frequency planning and allocation within such systems.
2. Description of the Related Art
The explosive growth of radio telecommunications technology in recent years and its utilzation by consumers has required continual improvement in the traffic capacity of cellular systems. For example, in order to improve the spectrum efficiency of cellular systems, the industry has moved rapidly from analog modulation techniques to digital modulation techniques. This has dramatically increased the number of simultaneous calls which can be handled by a cellular system on a discreet number of radio frequency channels. Time division multiple access (TDMA) systems such as the global system for mobile (GSM) communications allows a single radio frequency channel to be time divided into a number of separate time slots with one cellular subscriber's conversation being transmitted during each of the sequentially occurring time slots. This and similar techniques have greatly expanded the capacity of cellular systems.
One important procedure which is used in the management of cellular radio telecommunications systems in order to maximize the spectral efficiency of those systems is that of frequency reuse schemes. With such frequency reuse plans, the same radio frequency channel can be used simultaneously to carry different calls in different geographically separated areas of the system, known as cells. One constraint on the reuse of the same frequency for different channels within a cellular system is that the cells in which the same frequencies are used must be physically spaced far enough apart from one another that the interference between the two competing channels (referred to as co-channel interference) is low enough to provide acceptable voice quality for the users both of the channels. The level of co-channel interference must be balanced against the system operator's desire to reuse the same limited number of radio frequency channels as much as possible within the system.
The systematic reuse of radio frequency channels within the different cells of a cellular system must be carefully organized and planned for both existing traffic capacity as well as future expansion as traffic density within the system grows over time. Classically frequency reuse planning is done by a network operator by performing traffic and coverage analysis of the geographical area of interest and by determining the expected traffic load within that area of the system. Next, data are collected which includes the number of available frequencies, anticipated system growth, grade of service desired to be rendered to the subscribers in that area as well as population and mobile subscriber station distribution anticipated within the area. With these and other data, traffic calculations are performed to determine how many cell sites and cells are needed within the geographic area under consideration. From the number of frequencies which are available, the grade of service (GOS) desired to be rendered, Erlang tables and other parameters, the amount of traffic per anticipated subscriber may be calculated. Once a cell reuse pattern is selected, the operator can then determine the number of three sector cell sites based upon frequency reuse distances.
Many different cellular frequency reuse patterns are possible, however, the three major reuse patterns are 7/21, 4/12 and 3/9. In each of these three cases, the cell site geometry includes the following features:
(a) There are three cells (sectors) at each site. The antenna pointing azimuth of the cells are separated by 120.degree. and the cells are arranged with antennas pointing at one of the nearest site locations thus forming cells in a clover leaf fashion; PA1 (b) Each cell uses one 60.degree. radius transmitting antenna and two 60.degree. diversity receiving antennas with the same pointing azimuth; and PA1 (c) Each cell approximates the shape of a hexagon.
A group of neighboring cells using all of the channels which are available in the system, but not reusing them, is referred to as a cell cluster.
As can be seen, frequency planning has classically been a relatively complex and laborious process but one which is absolutely essential in order to accommodate growth within the system and enable an operator to maximize the utilization of the frequencies which it has available and, thus, maximize its investment in the system.
A great advantage could be obtained in the operation of a cellular system if an operator was able to automatically perform frequency allocation and reuse planning within its system on a continuous, and preferably automatic, basis. Automatic frequency allocation (AFA) would be a very desirable way to simplify frequency planning while still being able to obtain system capacity close to what is possible with manual implementation of multiple reuse patterns. The general idea behind automatic frequency allocation is that of monitoring within each cell the signal strength on all frequencies, or within a subset of all frequencies, which are available to the operator. The measured signal strength on each frequency is used to estimate the interference that would be generated if that frequency was to be used within that cell. If any of the frequencies which are non-allocated within a cell has a lower interference than that of a frequency which is allocated within that cell, a frequency switch is made. The most interfered with of the then allocated frequencies is replaced by the frequency having the lowest measured signal strength. This procedure is iteratively repeated until no further improvement in co-channel interference can be obtained within the cell.
In the performance of signal strength measurement for AFA, it can be argued that it is sufficient to measure the signal strength of various frequencies on the uplink only; that is, the signal strength of the frequency as received at the base station, since this would indicate which frequencies carry the traffic. However, there are two principal reasons why it is very important to measure the signal strength of each frequency also on the downlink; that is, on the radio signals as they are received at the mobile station. First, it is important to survey the interference environment within the interior of the cell where the traffic is actually located and not just at the periphery of the cell where the base station is located. Second, the base stations of most current systems utilize sectorized antennas which implies that the uplink signal strength measurements at the base station are incapable of estimating interference originating from traffic outside the antenna sector.
With respect to the first reason, most sectorized antennas are located on the periphery of the cell which they are serving. This means that the signals received along this periphery may not be fully representative of the actual radio traffic signal conditions within the environment where most of the traffic is occurring, i.e., out in the center in the cell as well as around the periphery of the cell at some distance away from the base station. Not only will buildings and other environmental obstructions change the signal levels for various frequencies within the cell but co-channel interference by the same frequencies reused in other cells may well be different around the periphery of the cell at some distance from the base station receiving antenna than they are contiguous to that antenna.
With respect to the second reason, the sectorized antennas at the base station only receive signals within the 120.degree. sector for which they are designed. Thus, if a particular signal is coming from outside of that sector, for example from an adjacent cell directly behind the sectorized antenna, it will measure a very low signal strength for the signal on that frequency but a mobile which is located in the middle of the cell and transmitting and receiving on an omni-directional antenna will detect a much higher signal strength on that frequency. It is this signal strength value which exists at the omni-directional antenna of the mobile station that creates co-channel interference and is thus most relevant with respect to frequency strength measurements useful in an automatic frequency allocation algorithm.
In digital cellular systems today, for example in the GSM system, downlink radio signal measurements are performed by the mobiles through a procedure known as mobile assisted hand-off (MAHO). While mobile stations which are in active mode may perform signal strength measurements on a large number of frequencies as instructed by the base station, they may only report on a small fraction of these. For example, when a GSM mobile is initially turned on and enters the idle mode it starts to find the strongest BCCH carrier of the frequencies stored in its subscriber information module (SIM) card. Once the idle mobile camps on that BCCH carrier it is periodically sent an idle mode BA-list on the system information type 2 carried on the BCCH carrier. The mobile uses this idle mode BA-list to measure the BCCH carriers among its currently serving base station and the base stations serving cells neighboring the one it is in to determine which has the strongest signal and, thus, which it should camp on for purposes of receiving or originating a call. Once the idle mobile becomes active and a call is being set up to or from it, the mobile is sent an active BA list on the slow associated control channel (SACCH) in the form of system information type 5. The frequencies on the active BA-list sent on the SACCH are those on which the mobile station should periodically measure the signal strength and send to the network in the form of a measurement report used to determine a hand-off candidate list.
The broadcast control channel (BCCH) is broadcast by the base station of the cell and includes information such as location area identity (LIA), the maximum output power allowed in the cell and the BCCH-carrier frequencies for the neighboring cells on which idle mobile stations are to perform measurements for possible cell reselection should the quality of the signal of the currently serving cell deteriorate. This list of BCCH carriers is called the idle BA-list and is sent on the BCCH in the form of a so-called, system information type 2 message. Once the mobile is active within a cell and maintaining communication on a traffic channel (TCH) it is periodically sent information on the SACCH in the form of messages from the network to the mobile stations within the cell. These messages give each mobile station updated information on the BCCH channel allocations in their neighboring cells by means of the system 5 information. These SACCH messages are broadcast to the mobiles by "stealing" time on the traffic channels. When the mobile receives the system 5 information blocks on the SACCH it may be in the form of a bit map identifying the particular BCCH channel frequencies of the neighboring cells upon which the mobile is to make signal quality measurements to be reported back to serving base station in a measurement report. Any change in the neighboring cell's description contained in the system 5 information on the SACCH is used to overwrite any old data held in the memory of the mobile station which it may have initially received on the BCCH when it entered active mode within the cell.
Once a mobile receives the, so-called, active BA-list in the system 5 information, it regularly measures the signal strength on each of the BCCH carrier frequencies of neighboring cells contained in the active BA-list. As part of this measurement, the mobile must also attempt to decode the particular base station identity code (BSIC) encoded into the signal being broadcast by each of neighboring base stations on its respective BCCH carrier. The mobile's ability to decode the BSIC information encoded into a neighboring base station's BCCH is used as one criteria of the quality of signal currently being received by the mobile from that neighboring base station.
In accordance with a standardized procedure, such as the GSM specification, once the mobile has made each of the measurements on the BCCH signals of the neighboring base stations in the active BA-list sent on the system 5 information, it then formulates a measurement report which is structured in a particular defined format. This format contains information on the six strongest BCCH carrier frequency signal measurements upon which the mobile was able to decode the BSIC. The information in the measurement report received from each mobile station is then used by its serving base station and the network to maintain a list of possible hand-off candidates for the base station in the event that the signal from a neighboring base is better than the signal quality of the signal of the mobile's currently serving base station. That is, the MAHO signal measurements made by a mobile within current cellular systems, such as the GSM system, is focused principally on measuring the signal quality of the BCCH carrier frequencies and reporting those back to the serving base station primarily for purposes of hand-off. Even though a mobile station will measure the signal strength on a TCH frequency included in the system 5 information, the mobile station will not include that measurement in the measurement report. This is because the mobile station will not decode any BSIC on a TCH carrier since no BSIC is transmitted on TCH frequencies. Although the capability exists within the mobiles for measuring the signal quality of any frequency it is instructed to measure, there is in neither the system 5 information messages a provision to tell a mobile station which frequencies in the active BA-list are TCH frequencies nor is there any facility within the measurement report format and structure for reporting to the base station the signal quality of any channels other than the BCCH channels included in the BA-list sent to the mobile.
In order to provide the ability within existing cellular system to use the mobile's measurement capabilities to measure traffic channel (TCH) signal quality and report its measurements on those traffic channels back to the base station for purposes of automatic frequency allocation, there needs to be both a structure and procedure for using such enhanced capabilities which can co-exist within the existing procedures. The method and system of the present invention provides such a capability.